With automatic transmissions known from practice, such as dual-clutch transmissions, so-called constant pumps, such as sickle-vane pumps, single or double-stroke vane cell pumps or the like, are provided for hydraulic supply, as is generally known. Such constant pumps are coupled with a fixed transmission ratio with a transmission input shaft, and are driven with the rotational speed of a drive unit of a vehicle drive train coupled to the transmission input shaft or to a rotational speed corresponding to this. As a result, as the rotational speed of the drive unit increases, the conveying capacity of the pump also directly increases. Starting from a comparatively low drive rotational speed of the hydraulic pump or the constant pump, the latter conveys a correspondingly high conveying volume flow, in order to supply the automatic transmission with hydraulic fluid in a sufficient scope and present a desired high performance, for example, during a shifting process of a vehicle.
Since the conveying capacity of constant pumps is to be designed in such a manner that an automatic transmission is supplied with sufficient hydraulic fluid by its hydraulic pump over its entire operating range, and, at low drive rotational speeds, a hydraulic pump conveys a volume of hydraulic fluid required for this, such a design of the hydraulic pump leads to the fact that, with increasing drive rotational speed, a constant pump conveys greater volumes of hydraulic fluid than is required for the supply of an automatic transmission. Such large volumes of hydraulic fluid conveyed by the hydraulic pump over wide operating ranges of an automatic transmission increases the power losses of automatic transmissions to an undesired extent, which in turn results in an increase in the energy consumption of a drive unit of a vehicle drive train, which can be at least one internal combustion engine, at least one electric motor or a combination of at least one internal combustion engine and at least one electric motor.
In order to be able to implement defined energy consumption targets of drive units to the extent required, adjustable hydraulic pumps, which are increasingly designed as controllable variable displacement pumps, are employed. Such pumps are designed with a correspondingly intelligent, software-driven activation function and a displacement volume that can be varied between a minimum value and a maximum value. If adjustable hydraulic pumps convey their defined maximum value, they are operating in their so-called “full stroke operation” and feature the conveying behavior of a constant pump.
A method for operating a hydraulic actuating device for a transmission with two hydraulic pumps, where at least the conveying capacity of one of the hydraulic pumps is variable, is known from DE 10 2014 207 798 A1. The described hydraulic pumps are controlled by an electromagnetic pressure regulator and, disadvantageously, feature in their overall assembly temperature-dependent production tolerances that are inherent in their principles and cause an increased actuating effort.
Through an adaptation described in the applicant's non-published patent application DE 10 2014 226 548.7 during a regulating operation of a variable displacement pump, during which the variable displacement pump or hydraulic pump may provide a higher conveying volume than is currently required, it can be ensured that the pump-side conveying volume flow, after saturation of an operating point-dependent leakage of a hydraulic control device of the automatic transmission and the indirect compensation of the inaccuracy caused by the control system also corresponds to the conveying volume flow, which is to be adjusted according to a request.
In this case, a so-called “self-leakage,” which is present in the area of the variable displacement pump, is itself compensated by a hydraulic circuit of the variable displacement pump. In full stroke operation, the variable displacement pump is operated with a corresponding self-leakage for which there is no compensation. Such an operating state of a variable displacement pump arises mainly if the hydraulic pump is driven by a drive unit that is designed as an internal combustion engine, the rotational speed of which currently features a so-called “idle speed level.”
If, in the area of the hydraulic system of a transmission, based on a request, such a high volume flow of hydraulic fluid is guided in the direction of a hydraulic load, which is higher than the volume flow of hydraulic fluid currently provided by the hydraulic pump, this represents a violation of the volume flow balance based on an actuation of the actuator system that is too rapid, which results in an over-excitation of the system. Such an over-excitation of the system in turn causes a collapse of the so-called “system pressure” of the hydraulic system in the direction of a pressure level, at which the behavior of clutch valves assigned to shifting elements, such as clutches, changes to the effect that it varies suddenly by a significant value. Such changes to the operating state in the area of the clutch valves produce, for the driver, reaction moments that are unexpected and impair driving comfort in the area of the vehicle drive train, which cause discontinuities in the progression of an output torque and jolts and shocks result from these discontinuities.
However, the limitation of the actuation gradient of the hydraulic system of a transmission is limited from physical aspects, since, for example, actuation sequences during a gear change are to be performed within defined operating times.
Therefore, during progressions of operating states, during which high leakage volume flows arise, the idle speed of the engine is increased by a correspondingly high value, in order to ensure that the adjustable hydraulic pump, during a full stroke operation or a constant pump, provides a flow of conveying volume that is required to prevent an under-supply state.
However, since, as is generally known, hydraulic pumps have certain production tolerances and, over their service lives, experience increased leakage volume flows in the area of a hydraulic pump because of wear, the so-called “increased idle speed” is selected to be correspondingly high, in order to, even during unfavorable progressions of operating states, convey the volume of hydraulic fluid required for avoiding under-supply operating states of a hydraulic system by an adjustable hydraulic pump.
However, this approach once again impairs the degree of efficiency of a vehicle, since higher rotational speeds bring about a higher energy consumption of drive units.
In contrast, hydraulic pumps of transmissions that have lower production tolerances and are operated to the same extent as hydraulic pumps of transmissions with higher production tolerances convey volumes of hydraulic fluid that are unnecessarily high, which in turn also impairs the overall degree of efficiency of a transmission. Therefore, there are attempts to keep the conveying volume of the hydraulic pump as low as possible. This requirement adversely affects driving comfort during unfavorable progressions of operating states. This is the case, for example, if the leakage volume flows resulting from the production tolerances and from increasing wear exceed the value underlying the design of the conveying volume in the area of a transmission designed with a hydraulic pump.